1. Field of the Invention
The present invention relates generally to a circuit and method for EFM demodulation, and more specifically, to a circuit and method capable of correcting data errors caused by invalid EFM bit patterns during demodulation by using alconversion table, based on fuzzy logic.
2. Prior Art
Data is stored on a compact disc as a single spiral of pits and lands. The data is encoded as eight-to-fourteen modulation or EFM. EFM is an encoding technique where 8-bit digital values are converted into 14-channel bit symbols. The basic block diagram of all compact disc players includes an EFM demodulation stage. The EFM demodulation stage can be implemented using a ROM look-up table or a logic array.
The standard EFM conversion table defines the conversion of 8 bit digital to 14-channel bit symbols. The assignment of 8-bit digital to 14-channel bit symbols called out by the standard EFM conversion table was done arbitrarily and the 14-channel bit symbols do not follow a specific sequential binary progression. Bit patterns in the standard EFM conversion table were chosen carefully to minimize the number of transitions required to represent the original 8-bit digital data making the compact disc easier to manufacture and to establish a minimum and maximum length of pits and lands. The number of possible patterns for a 14-bit binary number is 16,384 and the number of patterns used to represent EFM is 258 (256 for data and 2 patterns for sub-code synchronization purposes) which leave 16,126 invalid patterns unused and undefined by the standard EFM conversion table.
The compact disc player uses a laser head assembly to read the reflective contrast of pit versus land and convert that contrast into a high frequency wave that is sliced at the zero crossing point into a digital series of ones and zeros. The format of the compact disc defines the data to be grouped into 588-channel bit frames. Each 588-channel bit frame includes a 24-channel bit synchronization pattern; one 14-channel bit sub-code symbol, twenty-four 14-channel bit symbols of data and eight 14-channel bit symbols of error correction code. Three merging bits separate the synchronization pattern and each 14-channel bit symbol. The merging bits are used to maintain the minimum and the maximum pit and land lengths and to minimize the DC content of the signal over time.
The pits on a compact disc are among the smallest structures ever manufactured. A compact disc may have as many as 3 billion pits. Pits of nine separate lengths are used on the compact disc. The pits range in size from the shortest that is 0.833 micrometers to the longest that is 3.05 micrometers in length. All of the pits are approximately 0.5 micrometers wide and approximately 0.11 micrometers deep. It may be easier to visualize just how small the pits on the compact disc are if you consider that a human hair is approximately 75 micrometers in diameter. Precise manufacturing control of the shape, length, width and depth of the pits is required to make a disc playable. Variations in the manufacturing process of compact discs can cause problems such as jitter or variation of pit and land lengths, which may cause data errors during playback. Data errors are common place and error correction circuits are required and can be found in all compact disc players. Traditional error correction on the compact disc player occurs after EFM demodulation (fourteen bit symbols back to 8-bit digital values). Existing compact disc systems do not prevent invalid EFM 14-channel bit patterns from entering the system and creating data errors during translation into 8-bit digital values and rely solely on an elaborate multiple stage error correction scheme for providing error-free data.
Error correction in a compact disc system:carries a format burden of 8 symbols of error correction code for every 24 symbols of user data. This format burden affects both data transfer rate and overall storage capacity of the compact disc. The error correction system is precise and effective, but at the same time is limited in the number of corrections it can make per frame of data before it fails and an entire frame of data is lost. An improvement to the system, which reduces errors, will also reduce the chance that the error correction circuitry will reach its limit and fail. The ideal improvement of the compact disc error correction system would be one that increases the error correction capability without increasing the burden or creating incompatibility.
Fuzzy logic is a form of logic where absolute or one-to-one correspondence is not required to determine the correct or most likely choice. The demodulation of EFM offers an ideal opportunity for the application of fuzzy logic. The patterns selected for EFM tend to be as different as possible from each other. The fact that EFM uses only 258 out of 16,384 patterns leaves many invalid patterns, which are very similar to valid EFM patterns.
The integrity of data in digital systems is of crucial importance. A single uncorrected data error in a software program can cause an entire system to fail. The manufacturers of compact discs and compact disc players go to great lengths to create a system which can deliver error-free data under less than ideal conditions. The consumer""s demands for less expensive and faster methods to transfer data from the compact disc continues to drive the industry. Industry""s response to this demand has tested the compact disc specification based on a 1xc3x97 data rate. The original compact disc specification detailed how to build a disc and player for 1xc3x97 data rate, where present day compact disc drives commonly used in computer systems are now approaching data rates of 50xc3x97 and beyond.
Briefly, in accordance with the invention, a method for EFM demodulation is provided which includes a fuzzy logic-based ROM look-up conversion table capable of correcting data errors caused by invalid EFM bit patterns. The ROM look-up table contains the conversion data of valid EFM 14-channel bit patterns to 8-bit digital values. In addition to the valid pattern conversion, the ROM look-up table also includes fuzzy logic-based conversion of all invalid EFM 14-channel bit patterns to 8-bit digital values. The conversion of invalid EFM 14-channel bit patterns to 8-bit digital values allows error correction to begin during EFM demodulation using the inventive method.
Considering that there are 16,384 possible 14-channel bit patterns and EFM uses only 258 of those patterns, it is not surprising that a high percentage of data errors that occur in EFM tend to fall outside of the 258 valid 14-channel bit patterns. All data errors caused by invalid EFM 14-channel bit patterns have a chance of being corrected during conversion back to 8-bit digital values by the invention. The invention is not limited to how many symbols it can correct per frame as the traditional error correction circuitry. The invention improves the depth of the overall error correction system by correcting many of the data errors caused by invalid 14-channel bit patterns and most of the simple xe2x80x9coff by one clock periodxe2x80x9d invalid patterns. The inventive method relies on traditional error correction circuits for the errors that do not show up as invalid patterns or those that it is unable to resolve. If the inventive method is unable to correct an error caused by an invalid EFM 14-channel bit pattern, the attempt to correct the error is transparent to the preceding error detection and correction circuits. The error detecting and correcting circuits detect and correct the error just as if the invalid EFM 14-channel bit pattern has been demodulated by a traditional EFM demodulation circuit.
The inventive method is not limited to compact disc applications, but can be used in other systems such as digital versatile disc (DVD), digital audio tape (DAT) and other systems which use EFM or similar block encoding.
An alternate method and circuit to correct errors caused by invalid EFM 14-channel bit patterns is implemented by adding digital logic gates to the shift register stage of an EFM demodulator. A percentage of errors caused by invalid 14-channel bit patterns can be corrected by converting invalid channel bit patterns detected as 1T and 2T channel bit lengths to valid 3T channel bit lengths using digital logic gates in conjunction with the shift register stage of the EFM demodulator.
The alternate method and circuit will convert some of the invalid 14-channel bit patterns to valid 14-channel bit patterns before the parallel output of the shift register is used as the address to a standard EFM conversion table. Therefore, some errors caused by invalid 14-channel bit patterns will be corrected.
The digital logic circuits convert invalid 1T and 2T channel bit lengths to valid 3T channel bit lengths while the channel bits are shifted through the shift register in the EFM demodulator circuit.
The alternate method and circuit is simpler to implement than the method and circuit of the preferred embodiment. However, the alternate method and circuit does not attempt to correct all possible invalid 14-channel bit patterns and does not base it""s conversion of invalid patterns on a fuzzy logic comparison to valid 14-channel bit patterns. Therefore, the error correction performance of the alternate approach is inferior to the fuzzy logic-based look-up table described in the preferred embodiment.
The logic gates detect the occurrence of 1T pattern xe2x80x9c0110xe2x80x9d within the 14-channel bit symbol and convert the 1T pattern to a 3T xe2x80x9c1001xe2x80x9d pattern. The 2T pattern of xe2x80x9c1010xe2x80x9d is also converted to a 3T xe2x80x9c1001xe2x80x9d pattern by the digital logic gates.
It is therefore a principal object of the present invention to provide a circuit and method for EFM demodulation and which is capable of correcting data errors caused by invalid EFM bit patterns using a fuzzy logic-based ROM look-up table.
It is also an object of the present invention to improve the overall error correction capability by working in conjunction with the existing compact disc player error correction circuits without adding burden or incompatibility to the compact disc format. By improving the error correction capability of the error correction system, the resulting compact disc player will be more robust and fault tolerant.
It is another object of the present invention to allow the correction of errors caused by invalid EFM bit patterns to begin prior to the existing error correction circuits of the compact disc player and not add further delay (in comparison to traditional EFM demodulation).
It is still another object of the present invention to provide a method of EFM demodulation, which method can begin correcting errors caused by invalid EFM bit patterns before traditional error correction circuits that can be easily added to integrated circuit designs which currently include a traditional EFM demodulation stage.